1. Field of the Invention
The invention relates to a method and an apparatus for rounding off edges.
2. Description of the Prior Art
In many component parts that are under heavy loads mechanically, thermally or in other ways, all kinds of peak stresses occur that entail the risk of failure of the component. It has long been known to at least partly abate these peak stresses by making undercuts and/or radii and thus to shift the load limit of the component upward. In many components, proceeding in this way has its limits, since the requisite cutting tools, such as lathe chisels, radius milling cutters, grinding bodies or the like require a certain amount of space, and this need cannot always be met. In fuel injection systems, for instance, various heavily loaded components are present that are inaccessible, or accessible only at major effort, to the above tools.
In other components, such as injection nozzles, the presence of edges that are rounded off in a defined way is important, in order to keep the flow resistance of the component within a narrow tolerance range. For this group of components as well, there has until now been no method or apparatus for rounding off the edges that are definitive for the flow resistance with high replicability and at little cost.
From U.S. Pat. No. 5,807,163, a method for rounding off the edges of the tiniest drilled bores, such as injection ports of injection nozzles, and for calibrating the tiniest bores is known. So-called flow grinding with a polymeric plastic composition is also known. Both methods are unsuited to removing the macroscopic burrs, created in the production of common rails for fuel injection systems, economically and rounding off the edges that are present.
It is the object of the invention to furnish a method with the aid of which edges can be rounded off even at inaccessible places, with high replicability and a high rate of removal of material and at favorable cost.
According to the invention, this object is attained by a method for rounding off edges in which the edge to be rounded off is bathed by an erosive fluid; between the pressures of the erosive fluid before and after the bathing of the edge to be rounded off, there is a pressure difference of from 50 bar to 140 bar.
The method of this invention has the advantage that edges that may be present at virtually arbitrary places in components, even if the edges are of complicated geometry, can be rounded off. The expense for equipment is tolerable, the machining time is short, and the rounding-off quality is high. Furthermore, only slight costs are entailed. By continuously measuring the quantity of the erosive fluid, high process reliability is gained. The removal of material is the greatest at edges, because of what as a rule is an elevated flow speed, so that at the other places subjected to the erosive fluid, no removal of material or only slight removal of material occurs. The flow behavior of the erosive fluid, in components experiencing a flow through them, such as high-pressure fuel reservoirs of fuel injection systems, is equivalent to that of the fuel in operation, so that the rounding off according to the invention at the same time brings about a desired artificial aging of the component.
In a variant of the method of the invention, the flow speed of the fluid in the region of the edge to be rounded off is elevated compared to the average flow speed of the fluid, so that an especially large amount of removal of material is attained in the region of the edge to be rounded off.
In a further feature of the method, a body is introduced into the fluid, the surface of which body forms a gap with the edge to be rounded off, so that the flow speed of the fluid in the region of the edge to be rounded off is increased still further compared to the average flow speed of the fluid, and thus the removal of material is increased further as well.
In an expansion of the method, it is provided that the flow direction of the fluid and the longitudinal axis of the edge to be rounded off form an angle, in particular of 90xc2x0, thus further intensifying the removal of material.
It has proved to be especially advantageous to use a suspension of a grinding agent in oil as the erosive fluid. This suspension makes a greater removal of material possible, compared to the use of a polymeric plastic composition. In addition, cleaning of the workpieces once the method has been performed is simplified considerably. Finally, by the use of a suspension instead of a plastic composition, the fluid flow during the machining can be measured and monitored in a simple way. Since the fluid flow is correlated with the removal of material at the edges to be rounded off, the progress of workpiece machining can be monitored continuously on the basis of it. This is important above all in large- scale mass production of workpieces with extreme precision. The method of the invention is thus more economical and can be more widely used.
The object stated at the outset is also attained by an apparatus for performing the method of having a feed pump for the erosive fluid, and having a hydraulic communication between the feed pump and the component whose edge is to be rounded off. This apparatus has the advantage that because of the hydraulic communication of the feed pump and the component, a flow of the erosive fluid, above all in the region of the edge to be rounded off, can be built up, the consequence of which is the rounding off of the edge.
In a variant of the invention, it is provided that a body forming a gap with the edge to be rounded off is present, so that the flow speed of the erosive fluid is elevated in the region of the edge to be rounded off, compared to the average flow speed of the erosive fluid, thus speeding up the rounding off of the edge.
In an expansion of the invention, a collector device for catching the fluid is present, so that the erosive fluid does not reach the environment.
In another feature of the invention, the fluid is carried in closed loop circulation, thus reducing the consumption of erosive fluid.